THE ACTIVITY CYCLE OF SIGMA-DRACONIS

The temperature and granulation of several dwarf stars have been monitored since 1984. The results for sigma Dra (HR 7462, HD 185144, K0 V) are discussed in this paper. Temperature is monitored using a line-depth ratio, and temperature variations almost-equal-to 5 K are seen. Intermediate-band photometry and Ca II H and K-line emission mimic the temperature changes: a monotonic decline from the 1984 season, a smooth minimum around 1988, followed by a rise back to the 1984 values at the current time. These types of correlated changes are similar to the solar behavior. The full length of the cycle cannot yet be determined. The temperature variations are physically compatible with the photometric ones, implying a constant radius over the activity cycle. Granulation is invariant during this portion of the activity cycle, at least to the level of almost-equal-to +/- 3 m s-1 or about +/- 5%. These results confirm the importance of viewing the solar cycle within the context of stellar activity. The temperature variations by themselves can also be interpreted as rotational modulation with a 20.3 day period, but no evidence of this period is seen in the photometry, the H and K emission, or the line asymmetries, and it seems likely it is a chance occurrence. The 1985 season H and K observations show a significant period of 26.5 days, but again, this periodicity is not detected in other parameters.